That dreadful sin of unbelief

What can hinder your espousals with the dear and ever-blessed Lamb of God? I know but of one thing, that dreadful sin of unbelief. But this is my comfort, Jesus died for unbelief, as well as for other sins and has promised to send down the Holy Spirit to convince the world of this sin in particular. ‘If I go not away, the Comforter will not come unto you. But if I go away, I will send the Comforter and he will convince the world of sin.’ What sin? Of unbelief: ‘because they believe not on me.’